Monday, October 24, 2016

Scientists used data from NASA’s Solar Dynamics
Observatory, NASA’s Interface Region Imaging Spectrograph, and the Big
Bear Solar Observatory to track a solar wave as it channeled upwards
from the sun’s surface into the atmosphere. Credits: Zhao et al/NASA/SDO/IRIS/BBSO.Hi-res image

Scientists analyzed sunspot images from a trio of
observatories -- including the Big Bear Solar Observatory, which
captured this footage -- to make the first-ever observations of a solar
wave traveling up into the sun’s atmosphere from a sunspot. Credits: BBSO/Zhao et al

Tracking solar waves like this provides a novel tool for scientists
to study the atmosphere of the sun. The imagery of the journey also
confirms existing ideas, helping to nail down the existence of a
mechanism that moves energy – and therefore heat – into the sun’s
mysteriously-hot upper atmosphere, called the corona. A study on these
results was published Oct. 11, 2016, in The Astrophysical Journal Letters.

“We see certain kinds of solar seismic waves channeling upwards into
the lower atmosphere, called the chromosphere, and from there, into the
corona,” said Junwei Zhao, a solar scientist at Stanford University in
Stanford, California, and lead author on the study. “This research gives
us a new viewpoint to look at waves that can contribute to the energy
of the atmosphere.”

The study makes use of the wealth of data captured by NASA’s Solar
Dynamics Observatory, NASA’s Interface Region Imaging Spectrograph, and
the Big Bear Solar Observatory in Big Bear Lake, California. Together,
these observatories watch the sun in 16 wavelengths of light that show
the sun’s surface and lower atmosphere. SDO alone captures 11 of these.

“SDO takes images of the sun in many different wavelengths at a high
time resolution,” said Dean Pesnell, SDO project scientist at NASA’s
Goddard Space Flight Center in Greenbelt, Maryland. “That lets you see
the frequencies of these waves – if you didn’t have such rapid-fire
images, you’d lose track of the waves from one image to the next.”

Though scientists have long suspected that the waves they spot in the
sun’s surface, called the photosphere, are linked to those seen in the
lowest reaches of the sun’s atmosphere, called the chromosphere, this
new analysis is the first time that scientists have managed to actually
watch the wave travel up through the various layers into the sun’s
atmosphere.

When material is heated to high temperatures, it releases energy in
the form of light. The type, or wavelength, of that light is determined
by what the material is, as well as its temperature. That means
different wavelengths from the sun can be mapped to different
temperatures of solar material. Since we know how the sun’s temperature
changes throughout the layers of its atmosphere, we can then order these
wavelengths according to their height above the surface – and
essentially watch solar waves as they travel upwards.

The implications of this study are twofold – first, this technique
for watching the waves itself gives scientists a new tool to understand
the sun’s lower atmosphere.

“Watching the waves move upwards tells us a lot about the properties
of the atmosphere above sunspots – like temperature, pressure, and
density,” said Ruizhu Chen, a graduate student scientist at Stanford who
is an author on the study. “More importantly, we can figure out the
magnetic field strength and direction.”

The effect of the magnetic field on these waves is pronounced.
Instead of traveling straight upwards through the sun, the waves veer
off, taking a curved path through the atmosphere.

“The magnetic field is acting like railroad tracks, guiding the waves
as they move up through the atmosphere,” said Pesnell, who was not
involved in this study.

The second implication of this new research is for a long-standing question in solar physics – the coronal heating problem.

The sun produces energy by fusing hydrogen at its core, so the
simplest models suggest that each layer of the sun should be cooler as
you move outward. However, the sun’s atmosphere, called the corona, is
about a hundred times hotter than the region below – counter to what you
would expect.

No one has as-yet been able to definitively pinpoint the source of
all the extra heat in the corona, but these waves may play a small role.

“When a wave travels upwards, a number of different things can
happen,” said Zhao. “Some may reflect back downwards, or contribute to
heating – but by how much, we don’t yet know.”

NASA Goddard built, operates and manages the SDO spacecraft for
NASA's Science Mission Directorate in Washington. Lockheed Martin
designed the IRIS observatory and manages the mission for NASA. The Big
Bear Solar Observatory is operated by the New Jersey Institute of
Technology in Newark, New Jersey.